Upgrading to Vega would be considered the same as upgrading to the Ti, since they are to be considered equivalent according to rumored specs.Just for 30% gains and being notoriously expensive is not worth. I guess Vega or Volta seems like a good choice. Upgrading to newer card of the same generation will see only measly gains and not worth it.
As for Volta, I might consider it, even though it will only be a "refresh" as compared to Pascal (no fabrication node change, only optimizations and new VRAM type adoption, most possibly HBM2) so there still will not be any significant gains from it. (not as significant as Pascal vs Maxwell, that is)
If I do upgrade to either the Ti or Volta, then I will not upgrade again until 2160p60 becomes mainstream on mid-range hardware.
I really can't wait for that moment, as is apparent from my constant repeating of that same sentence. :F